During the 1980s Raman Scattering in fibers was demonstrated by Lin, Stolen, and other co-workers of AT&T Bell Laboratories in Holmdel, N.J., using lasers operating between 0.3 μm to 2.0 μm. In the early years of the Raman fiber before extensive work had begun, no one perceived that a Raman fiber could be pumped by a practical semiconductor laser-based source or that an efficient CW-pumped Raman Fiber Laser was possible.
However, with the development of Cladding-pumped Fiber Lasers and Fiber Bragg Gratings, diode-laser-based CW Raman Fiber Lasers have been made efficient, emitting at various wavelengths throughout the infrared spectrum a reality. (See van Gisbergen et al., (1996) Chem. Phys. Lett. 259: 599-604.)
Raman spectroscopy is a powerful optical technique for detecting and analyzing molecules. Its principle is based on detecting light scattered off a molecule that is shifted in energy with respect to the incident light. The shift, called Raman shift, is characteristic of individual molecules, reflecting their vibrational frequencies like molecular fingerprints. As a result, the key advantage of Raman spectroscopy is its molecular specificity while its main limitation is the small signal due to low quantum yield of Raman scattering. One way to enhance the Raman signal is to tune the excitation wavelength to be on resonance with an electronic transition, so called resonance Raman scattering. This can usually produce an enhancement on the order of 102-103.
Another technique to enhance Raman scattering is surface enhancement by roughened metal surfaces, notably silver and gold, that provides an enhancement factor on the order of 106-108. This is termed surface enhanced Raman spectroscopy (SERS). Similar or somewhat larger enhancement factors (˜108-1010) have been observed for metal, mostly silver or gold, nanoparticles.
In the last few years, it has been shown that an even larger enhancement (˜1010-1015) is possible for aggregates of metal nanoparticles (MNPs), silver and gold. The largest enhancement factor of 1014-1015 has been reported for rhodamine 6G (R6G) on single silver nanoparticle aggregates. This huge enhancement is thought to be mainly due to significant enhancement of the local electromagnetic field of the nanoparticle aggregate that strongly absorbs the incident excitation light for the Raman scattering process. With such large enhancement, many important molecules that are difficult to detect with Raman normally can now be easily detected. This opens many interesting and new opportunities for detecting and analyzing molecules using SERS with extremely high sensitivity and molecular specificity.
SERS can also be developed into a molecular imaging technique for biomedical and other applications. Existing Raman imaging equipment should be usable for SERS imaging. SERS will provide a much-enhanced signal and thereby significantly shortened data acquisition time, making the technique practically useful for medical or other commercial and industrial applications including chip inspection or chemical monitoring. SERS is also useful for detecting other cancer biomarkers that can interact or bind to the MNP surface. For example, Sutphen et al. have recently shown that lysophospholipids (LPL) are potential biomarkers of ovarian cancer (Sutphen et al., (2004) Cancer Epidemiol. Biomarker Prev. 13: 1185-1191).
For many practical applications, for example SERS and optical filters, it is highly desirable to narrow the distribution of size/shape of nanoparticle aggregates. For SERS in particular, the incident light has to be on resonance with the substrate absorption. Only those nanoparticle aggregates that have resonance absorption of the incident light are expected to be SERS active. It is thus extremely beneficial to have a narrow size/shape distribution and thereby narrow optical absorption.
Fluorescent nanoparticles (quantum dots (QDs) such as semiconductor quantum dots, SQDs) have been used recently as fluorescent biological markers and have been found to be extremely effective. They offer advantages including higher stability, stronger fluorescence, tunability of color, and possibility of optical encoding based on different sized or colored SQDs.
Metal nanoparticles have been recognized for their unique optical properties that could be exploited in optoelectronic devices. Nanoparticle systems composed of gold, for example, have distinct optical properties that make them amenable to study by Raman scattering. The Raman spectrum of the adsorbed species is significantly enhanced by 10 to 15 orders of magnitude when the metal nanoparticles have aggregated, leading to enhanced electromagnetic field effects near the surface that increases the Raman scattering intensity. The greater sensitivity found in the SERS of metal nanoparticle aggregates facilitates the detection and analysis of a whole host of molecules that were previously difficult to study.
Wang et al. disclose a method of using SQDs (dye-conjugated CdTe nanoparticles, CT-NPs) to detect interactive binding between Ag-CT-NPs and Ab-CT-NPs (Wang et al., (2002) NanoLett. 2: 817-822). The interactions were determined by differential quenching or enhancement fluorescence activity of two different sized SQDs (red or green) measured during the analysis.
The use of SERS for analyte detection of biomolecules has been previously studied. U.S. Pat. No. 6,699,724 to West et al. describes a chemical sensing device and method (nanoshell-modified ELISA technique) based on the enzyme-linked immunoadsorbant assay (ELISA). The chemical sensing device can comprise a core comprising gold sulfide and a surface capable of inducing surface enhanced Raman scattering (SERS). In much of the patent disclosure, the nanoparticle is disclosed as having a silica core and a gold shell. The patent discloses that an enhancement of 600,000-fold (6×105) in the Raman signal using conjugated mercaptoaniline was observed.
In the nanoshell-modified ELISA technique, antibodies are directly bound to the metal nanoshells. Raman spectra are taken of the antibody-nanoshell conjugates before and after the addition of a sample containing a possible antigen, and binding of antigen to antibody is expected to cause a detectable shift in the spectra.
The conjugation of quantum dots to antibodies used for ultrasensitive nonisotopic detection for use in biological assays has also been studied. U.S. Pat. No. 6,468,808 B1 to Nie et al. disclosed an antibody is conjugated to a water-soluble quantum dot. The binding of the quantum dot-antibody conjugate to a targeted protein will result in agglutination, which can be detected using an epi-fluorescence microscope. In addition, Nie et al. described a system in which a quantum dot is attached to one end of an oligonucleotide and a quenching moiety is attached to the other. The preferred quenching moiety in the Nie patent is a nonfluorescent organic chromophore such as 4-[4′-dimethylaminophenylazo]benzoic acid (DABCYL).
Raman amplifiers are also expected to be used globally as a key device in next-generation optical communications, for example, in wavelength-division-multiplexing (WDM) transmission systems. Raman scattering occurs when an atom absorbs a photon and another photon of a different energy is released. The energy difference excites the atom and causes it to release a photon with low energy; therefore, more light energy is transferred to the photons in the light path.
Improving the consistency of SERS probes requires the use of single, SERS active nano-sized structures. Nano-crescents, and core-shell systems are examples of cleverly engineered nanostructures capable of providing sufficient SERS intensity from individual particles due to their ability to strongly localize surface electromagnetic fields. (See in particular, Lu, Y., Liu, G. L., Kim, J., Mejia, Y. X., and Lee, L. P., Nano Lett. 2005, 5, 119-124; Talley, C. E., Jackson, J. B., Oubre, C., Grady, N. K., Hollars, C. W., Lane, S. M., Huser, T. R., Nordlander, P., and Halas, N. J., Nano Lett. 2005, 5, 1569-1574.) However, the relatively large size of these nanostructures will ultimately limit their accessibility to some sub-cellular organelles. To push the size boundary of sensing, as required by systems biology, even smaller probes will be required. Of interest is a subset of core-shell structures, hollow metal structures, a unique class of nanomaterials explored, most notably, by Sun et al. (Sun, Y. G., Mayers, B., and Xia, Y. N., Advanced Materials 2003, 15, 641-646). Utilizing the galvanic replacement of silver with gold and other metals, they have produced a variety of different sized and shaped hollow structures and have recently demonstrated the SERS activity of these structures (Chen, J. Y., Wiley, B., Li, Z. Y., Campbell, D., Saeki, F., Cang, H., Au, L., Lee, J., Li, X. D., and Xia, Y. N., Advanced Materials 2005, 17, 2255-2261).
In solid spherical particles there is a single resonance at approximately 520 nm for gold and 400 nm for silver, varying slightly depending on size and embedding media. However, when one axis is extended, for example, a nanorod, the resonance will break into two absorption bands, one corresponding to the short axis, or transverse mode, and another to the long axis, or longitudinal mode (Nikoobakht, B. and El-Sayed, M. A., Chem. Materials 2003, 15, 1957; Chang, S. S., Shih, C. W., Chen, C. D., Lai, W. C., and Wang, C. R. C., Langmuir 1999, 15, 701). The longitudinal mode has lower energy or redder absorption than the transfer mode. This is also true for aggregated systems in which there are multiple resonances within each given cluster of particles (Grant, C. D., Schwartzberg, A. M., Norman, T. J., and Zhang, J. Z., J. Am. Chem. Soc. 2003, 125, 549; Quinten, M. J., Cluster Sci. 1999, 10, 319; Quinten, M., Applied Physics B-Lasers and Optics 2000, 70, 579; Quinten, M. and Kreibig, U. Applied Optics 1993, 32, 6173; Norman, T. J. Jr. Grant, C. Magana, D. Cao, D. Bridges, F. Liu, J. van Buuren, T. and Zhang, J. Z., J. Phys. Chem. B 2002, 106, 7005; Norman, T. J., Grant, C. D., Schwartzberg, A. M., and Zhang, J. Z., Opt. Mat. 2005, 27, 1197; and Kreibig, U. Optical properties of metal clusters; Springer: Berlin; N. Y., 1995; Vol. 25). Therefore, controlling size and shape of these metal nanostructures allows control of their optical properties that have potential applications in nanophotonics and sensing.
As an effort to engineer so-called “hot spots” of large enhancement in single particles, Lee et al. produced nano-crescent structures by depositing silver over latex beads on a surface, then dissolving away the bead (Lu, Y., Liu, G. L., Kim, J., Mejia, Y. X., and Lee, L. P., Nano Lett. 2005, 5, 119). These hollow spheres are open-ended with a sharpened edge that greatly enhances the EM field. This engineered “hot-spot” approach yields improved SERS enhancements over core/shell systems and is of a similar homogeneity due to the highly consistent latex beads available. For applications requiring extremely small probe size, however, both nano-crescents and core shell systems are relatively large.
A system of particular interest where probe size is of utmost importance is intracellular studies (Chithrani, B. D., Ghazani, A. A., and Chan, W. C. W., Nano Lett. 2006, 6, 662-668). It has been found that while particles larger than 100 nm can enter a cell, they do not do so readily and may interrupt some cellular functions. Similarly, particles that are too small, less than 20 nm, will diffuse out of the cell, rendering them useless. The ideal is a structure that can be tuned in size between 20 nm and 100 nm depending on the application.
Nanotubes of all shapes and sizes have become an area of increasing interest for applications ranging from filtration to electrical interconnects. (See, in particular, Holt, J. K. et al., Science 312, 1034-1037 (2006); Hinds, B. J. et al., Science 303, 62-65 (2004); Zhang, M. et al. Science 309, 1215-1219 (2005); and Huang, Y. et al., Science 294, 1313-1317 (2001).) The application of these structures is almost unlimited, however, as is the case with most synthesized structures of this scale, nanoscopic manipulation is challenging. While carbon nanotubes have been the predominant structure of interest, lately there has been an effort to utilize gold and silver nanotubes or nanowires for these purposes as their conductivity and material properties are thought to be superior (Siwy, Z. et al., J. Am. Chem. Soc. 127, 5000-5001 (2005); Kohli, P., Wharton, J. E., Braide, O. & Martin, C. R. J., Nanosci. Nanotechnol. 4, 605-610 (2004)). Generally these metal structures are produced by a physical or electroless deposition technique, and while this produces well defined structures, their shape and size is entirely dependent on the template on which they are made, limiting the size and practical application of these structures (Wiley, B., Sun, Y. G., Mayers, B. & Xia, Y. N., Chemistry-a European Journal 11, 454-463 (2005); Wiley, B. et al., M.R.S. Bull., 30, 356-361 (2005); Sun, Y. G. & Xia, Y. N. Advanced Materials 16, 264-268 (2004); and Lee, M., Hong, S. C. & Kim, D., Appl. Phys. Lett., 89 (2006)).
There is therefore a need in the art for use in the chemical and biomedical analytical industries and the electronic communications industries to provide more sensitive compositions and devices that are inexpensive to manufacture and easy to use.